Radio Frequency IDentification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field. The RF wave may encode one or more commands that instruct the tags to perform one or more actions.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Accordingly, when a reader reads a tag code, data can be learned about the associated item that hosts the tag, and/or about the tag itself.
An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or semi-active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags.
Components of the RFID tag may require a voltage for proper operation. If the voltage provided varies, components of the RFID tag may not operate properly. Accordingly, a voltage regulator may be provided to regulate the provided voltage and reduce variations in the voltage. Conventional regulators may include an operational amplifier with feedback. The regulated voltage is fed back to the operational amplifier, which adjusts the regulated voltage based on a reference voltage. Such a feedback approach can be slow to respond to changes in the provided voltage because it takes time to receive the feedback voltage and make the output adjustment.
A source follower circuit 100 may also be used as a voltage regulator, as shown in FIG. 1. A current source Iref 102 is coupled to the drain of the transistor 105 and provided to bias the transistor 105. The transistor 105 is biased to be in saturation. A voltage source 110 having a value VREF—REG is coupled between the source of the transistor 105 and ground, establishing the voltage at the drain of transistor 105. With the gate and drain of transistor 105 coupled together and the current source Iref supplying current, the transistor 105 is forced into the saturation region of operation. The voltage at the gate, node 115 in FIG. 1, is greater than VREF—REG by the threshold voltage of the transistor 105, which may be approximately 500 or 600 mV in one example. The gate of the transistor 105 is coupled to the gate of a second transistor 120, which may be larger or smaller than the transistor 105 by a factor m. The voltage on the gate of transistor 120, generated by the transistor 105, controls the conductivity of the transistor 120 to generate a regulated voltage, VDD—REG as shown in FIG. 1, at the source of the transistor 120. The regulated voltage may be supplied to a load, represented in FIG. 1 as load current 125 and capacitance 130.
Using the voltage regulator of FIG. 1, the supply voltage VDD must be greater than the voltage VREF—REG by at least a threshold voltage of the transistor 105, which as described, may be approximately 500 or 600 mV. This extra 500 or 600 mV above the desired regulated voltage may be referred to as “overhead,” which is desirable to be minimized for low-power operation.